In case the event or specific jobs or shifts are not public, registration links can be created. The registration via these links is always possible.
Just create a new link and select some shifts. You can change the shifts also after the link was sent. Of course it is also possible to include a publicly visible job into a link.
The usage field is just for internal use to remember why a link was generated.
Links can only be created by admins, not by job admins.